This invention relates in general to spectrophotometers and, in particular, to a dual wavelength converter for a dual beam instrument.
A spectrophotometer is an instruement for measuring light transmitted through a substance. Based upon such measurements, properties of the substance can be determined, for example, concentration.
There are two types of spectrophotometers. One type is called a split beam or a dual beam spectrophotometer. Such spectrophotometers consist of a single monochromator light source of a given wavelength or wavelengths. The light from the source is divided into two separate beams that pass along separate optical paths. One optical path leads one beam through a reference; the other optical path leads the other beam through a sample. The reference does not contain any material but is used to measure or compare the sample beam against a common background. As such, the reference beam represents no change, 100 percent light transmission and zero absorption. In contrast, the sample will absorb some light and transmit the rest. The two beams are alternately transmitted along their respective paths. These paths terminate in a common photodetector. The photodetector includes a processing network for separately processing the alternate signals received at the detector from the reference path and the sample path.
Dual or split beam spectrophotometers are widely used because they are relatively inexpensive and provide reliable spectrophotometric information for many applications. However, they have certain shortcomings when measurements are undertaken on samples having high turbidity that tends to scatter rather than absorb incident light.
The other type of spectrophotometer is known as a dual wavelength instrument. Dual wavelength spectrophotometers use two, independent beams of light generated by two separate and distinct monochromators set at different wavelengths. These two beams of light are alternately transmitted along optical path or paths to the sample material. Electronic circuits measure the difference in absorption between the two beams. Information may be derived based upon the different output depending upon the relative difference in wavelengths of the two incident beams. Unlike the dual beam spectrophotometer, the dual wavelength spectrophotometer does not include a reference optical path. Instead, both of the distinct wavelength beams are transmitted to the sample.
Dual wavelength spectroscopy is useful, if not essential, when analyzing a turbid sample, but can also measure a clear, transparent sample. Dual wavelength spectroscopy is also useful in situations where a reference system/cuvette is not available, such as looking at whole tissue in organisms where there is no true identical reference, where a reference system may change in a much different kinetic manner than the sample, where material is scarce and not available for a reference, or where it is impossible or undesirable to have a reference, such as measurement of light in inaccessible places (a form of remote sensing). For example, when one measures the absorbance of hemoglobin in a living organism's artery, there is no reference. With dual wavelength spectroscopy, a "reference" signal is obtained from the same sample as the "sample" signal, but at a different wavelength. The so-called reference wavelength chosen is one where the compound or effect being measured has no absorbance. At that reference wavelength the sample acts as its own reference compared to the measuring wavelength.
The dual wavelength spectrophotometer is inherently more expensive than the dual beam spectrophotometer since the dual wavelength spectrophotometer necessarily includes more monochromators. Others have recognized that it would be desirable to combine the two instruments into one. As such, there are several examples of dual wavelength spectrophotometers that may be adapted to operate as more simpler, split beam spectrophotometers. See, for example, U.S. Pat. Nos. 3,927,944; 3,924,949; and 3,712,738. See also, "Sensitive Absorption Spectrophotometer for Use As A Split Beam or As a Dual Wavelength Instrument," R. Rikmenspoel, Review of Scientific Instruments, Vol. 36, No. 4, April 1965, Pages 4, 9, 7-503.
Despite the adaption of dual wavelength spectrophotometers to operate as dual beam instruments, there remains an unfulfilled need for adapting dual beam instruments to operate as dual wavelength instruments. There are believed to be a larger population of dual beam machines in existence than there are dual wavelength machines. Users of such dual beam machines on occasion may require the more sophisticated measurement techniques available only in dual wavelength machines. Hence, such users would desire a means for adapting the less expensive dual beam machines to occasionally operate as dual wavelength machines.